Watch these classic comedians use "the mirror gag," where one character pretends to be a reflection in a mirror and imitates another. Sometimes the gag's deceptive because one character is too stupid to realize it's not his reflection, and other times the two characters are just playing around with each other.

At first, they act within the rules of the game: they've got to imitate the other person, ignore that the imitation isn't perfect, and not break the plane of the mirror. But soon, the rules get broken.